Process for controlling electroplating



Dec. 16, 1930.

R. J. PIERSOL PROCESS FOR CONTROLLING ELECTROPLATING Filed Jan. 14. 1929INVENTOR Patented Dec. 16, 1930 UNITED STATES Romans .1. Hanson, orrrrrsnunen, rnimsnvnnm PROCESS FOR CONTROLLING ELECTROPLATINGApplication filed January 14, 1929. Serial at. 332,396.

This invention relates to electro-plating and particularly to a processfor maintaining a plating solution in continuously operative condition,and for maintaining the plating '8 action constant.

In electro-plat-ing, the chemical action, or

ionization, which accompanies the transfer of current through theplating solution,

causes the evolution of hydrogen by decomposing the water content of thesolution. The hydrogen collects at the surface of the cathode, orarticle being plated, and oxygen at the anode. The greater part of thehydrogen escapes from the solution, but some hydrogen remains andconstitutes a thin film on the cathode surface, establishing a conditionknown as polarization. The film of hydrogen acts as an insulating layer.

The decomposition voltage of water is less than the plating voltage of asolution.

In the electro-plating of chromium, the high current density used causesextreme liberation of hydrogen which builds up the film of insulation onthe cathode. and decreases the current for a given voltage. After anaccumulation of the gas builds up to relative thickness, it suddenly iscarried ofl by gassing.

The periodic change in the voltage and the current between the anode andthecathode has been observed as a characteristic of a chromium platingsolution. The duration of such change varies with platingconditions, Forexample, the voltage may start at 6 volts with a current of 100 amperesfor '20 seconds, and then drop suddenly to a voltage of 4.5 volts with acurrent of 60 amperes. After a similar period of about seconds, more orless, the voltage jumps up to the previous value with a correspondingincrease in current.

One explanation of this action is that chromium has anactive and apassive state, with different solution voltages, the difference betweenthem being about 1.8 volts. Above a certain current density, theevolution of free active hydrogen is very intense, throwing the chromiuminto the active state. 50 The other explanation of this action is thatit is dueto the hydrogen polarization of the cathode with the periodicrelease of the hydrogen from the cathode surface.

Irrespective of the cause of the fluctuations, it is desirable toeliminate such fluctuations in order to establish constant current andplating conditions.

An object of my invention, th refore, is to provide a process forcounter-ating the effects of polarization in a plating solution, wherebythe solution may be m "ntained continuously operative and the latingaction evenand steady.

Another object of my invention is to provide a, process 'for electrodepositing chromium, which shall establish an even steady plating actionand obviate the fluctuations heretofore observed in the volt-age andcurrent conditions in. chromium-plating solu- .tions.

I have found that a modification of the normal direct plating current ofconstant value into an undulating direct current, so modifies theplating action as to cause it to be steady and even, and entirely freeof the periodic changing of conditions heretofore referred to.

The normal plating voltage range for chromium as now generally employedis from 6 to 12 volts. With a current modified into an undulatingcurrent, I have been regularly and successfully plating with voltages aslow as 2.8 volts.

The modified plating voltageis an undulating or periodically varyingdirect voltage whose value varies about 1 volt above and below a mean oraverage voltage. The periodic recurring variation of abouttwo voltsbetween the maximum and minimum values of the modified plating voltageconsiderably changes the evolution of hydrogen at the cathode. Theresult appear to be a resonant or whipping action which induces regularand steady escape of the hydrogen from the cathode surface. The hydrogenis thus discharged from the cathode surface before a substantial filmmay collect.

. In order to obtain a current of the character described, I rectifyboth waves of an which alternating current energy is changed to directcurrent energy for the plating circuit by rectification; and Fig. 2shows a graph of the undulating nature of the plating current.

As is illustrated in Fig. 1 of the accompanying drawings, the platingsystem comprises a source 1 or supply circuit of alternating current, ofnormal commercial voltage, which energizes-a transformer 2 to supply alow alternating voltage approximating plating voltage, which is thenrectified by rectifying means 3 and supplied to plating solution 4.

The transformer may be provided with tap connections 5 or other suitablemeans for varying the derived voltage available for the plating action.Such connections may be provided on the primary or the secondarywinding, or both, in such number as to provide the gradationthat may bedesired;

In order to control the extent of undulation or variation of the derivedcurrent, a variable reactor 6 is provided for the plating circuit. Thesteps whereby the varying plating current is derived are illustrated inFigures 2, 3 and 4. In Figure 2 is illustrated the sine wave of thealternating current derived from the secondary winding of thetransformer. When rectified by the rectifying means, the current wouldbe substantially as illustrated in Figure 3 and sosupplied to theplating solution. v

The rectifying means may be of the mercury are or thermionic types, inwhich current is transmitted practically in one direction only, or itmay be of the electrolytic or metal oxide types in which a smallnegative current is transmitted. If rectifying means of thefirst-mentioned types are employed the rectified waves will besubstantially as shown in Figure 3. If rectifying means of thesecond-mentioned types are employed, the form of the rectified waveswill be modified somewhat by the small negative components, which willdimnish the resultant amplitude of the ositive'waves. The curve inFigure 3 may't erefore be considered as rep.- resenting the general formof resultant rectified waves of an alternating current.

By means of the inductance of the reactor 6, the rectified waves ofFigure 3 are smoothed out to establish a plating current as illustratedby curve A in Figure 4.

The amount of inductance shoud be suflicient to prevent the minimumvalue of the fluctuations of the derived plating current from diminishinto a value less than the contact-Volta e 0% example, as i lustrated inFigure 4, the line C represents the contact voltage of the solu tion.Below this value of impressed voltage, on a chromium solution,forexample, the back voltage tends to establish an electric cell actionand throw the deposited chromium back into solution. Since the rate ofdissolution of chromium, for example, greatly exceeds the rate ofdeposition, the variation in the plating voltage must not be sufiicientto reduce the lating voltage below the contact voltage of t e solution.

The magnitude of the variations or fluctuations in' the plating circuitmay be varied by adjusting the reactor 6. Where a special transformer isdesigned and provided for a particular system, a suitable amount ofinductive reactance maybe provided in the transformer itself.

By means of the variation in the plating current, the rate of evolutionof hydrogen is varied frequently and suddenly, and the gradualaccumulation ofhydrogen at the cathode surface is obviated. Thetroublesome sudden variations in plating conditions caused by suchaccumulations are likewise obviated.

Moreover, the necessity for higher plating voltages is also obviated,since the resistance of the hydrogen does not have to be overcome.

My invention is not limited to the specific details illustrated sincethe same action may be obtained without departing from the spirit andscope of the invention as set forth in the appended claims.

I claim as my invention:

1. A plating system including a plating solution having two electrodestherein, and means for supplying a plating current there to comprising asource of alternating current, a transformer having its primary windingconnected to said source, and rectifying means connected between thesecondary winding of the transformer and the electrodes .in the platingsolution, and means for controlling the amplitude and the phasecondition of the current supplied to the rectifying means.

2. A system as in claim 1, in which the rectifying means are of themetal-metaloxide type.

3. A system as in claim 1, in which means are provided to modify theform of the rectified waves toestablish a resultant undulating current,whose minimum value does not diminish below the contact voltage of thesolution. i

4. 'A system for electro-plating including a solution having twoelectrodes, a source of alternating current, rectifying means forconverting the alternating current to direct ourthe plating solution.For

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rent, means for variably modifying the current to be rectified in orderto vary the range of the undulating portion of the rectified current,means for controlling the amplitude of the rectified current, and meansfor supplying the rectified current to the electrodes in the solution.

5. A plating system including a plating solution having two electrodestherein, a source of alternating current, a transformer having itsprimary winding connected to the alternating source, rectifying meansconnected between the secondary winding and the electrodes in thesolution, and variable means including inductive means for controllingthe amplitude and phase relation of the current supplied to therectifying means to control the amplitude of the rectified current andthe nature of the undulating portion of the rectified current.

In testimony whereof, I have hereunto subscribed my name this 20th dayof December, 1928.

ROBERT J. PIERSOL.

